C++ template and sizeof

dmail · 08-02-2006, 11:36 AM

Whilst trying to break introuble post I came across something which I can not explain.
Enums are used alot in templates and the following struct baffles me

Code:

template<int size>
struct Amount
{
	enum{ value = size};
};

Now an enum is just an int or some type which is big enough to hold the value. Is this correct?

See if I do a size of

Code:

...
sizeof(Amount<60>)
...

The result is 1 byte so this got me thinking a char is being used; but if a sizeof is done on Amount<60>::value then 4 bytes is returned (size of an int). This is where I am really confused, the value inside the struct is bigger than the struct itself.

I then give the "size" a bigger value than what a char can hold (Amount<500>) and the same results were returned.

Have I just completely missed something here, or is this correct?

edit:
The results are the same for Kdevelop and Visual Studio so I am therefore assuming that it is correct and I have missed something.
Any ideas?

mldqj · 08-03-2006, 01:32 AM

There is nothing wrong. You defined an enum inside a struct. That enum is a nested type instead of a member variable of the struct. Therefore, struct Amount is empty in the sense that it does not hold any data.

dmail · 08-03-2006, 08:01 AM

Thanks for that mldqj.
By adding a data member to the struct I see the nested enum has no size at all.